Distributed optical fibre sensor

ABSTRACT

There is disclosed a distributed optical fiber sensor arranged to deliver probe light pulses of different wavelengths into corresponding different sensing optical fibers, and to determine one or more parameters as functions of position along each of the sensing fibers from detected backscattered light of each corresponding wavelength. In another arrangement, the different wavelengths are directed in different corresponding directions around a loop of sensing optical fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 13/992,578, filed onJun. 7, 2013, which is the national phase under 35 U.S.C. §371 ofInternational Application No. PCT/GB2011/052409, filed on Dec. 6, 2011,which claims priority to and the benefit of GB 1020827.0, filed on Dec.8, 2010, the entire contents of each of which are incorporated byreference herein.

The present invention relates to distributed optical fibre sensors, inwhich one or more physical parameters are sensed as a function ofposition along a sensing optical fibre from the properties of probelight backscattered within the sensing fibre. In particular, but notexclusively, the invention relates to optical time domain reflectometry(OTDR) sensors for use in sensing vibration, and such sensors which usephase sensitive OTDR techniques such as through the detection ofcoherent Rayleigh noise, or other interferometric techniques.

INTRODUCTION

Distributed optical fibre sensing is a well known approach to providinginformation about environmental conditions surrounding a sensing opticalfibre. Fully-distributed sensing in principle provides spatiallyresolved information from every point along the fibre. Variables thatcan be sensed include temperature, static strain, pressure, andvibration.

One such technique detects variations in refractive index, induced by aphysical forcing such as vibration, in the coherent Rayleigh noiseprofile of light backscattered within a sensing optical fibreinterrogated by an optical source of limited bandwidth. Such Rayleighnoise profiles arise from interference between the many components ofthe backscattered light originating from different points along aportion of the sensing optical fibre illuminated by the optical source.Such techniques are described, for example, in WO2008/056143.

Another such technique detects variations in fibre temperature andstrain through their effects on Brillouin scattering of probe lightlaunched into the fibre. Such techniques are described, for example, inHoriguchi et al., Journal of Lightwave Technology, vol. 13, no. 7, July1995.

It would be desirable to address problems and limitations of the relatedprior art.

SUMMARY OF THE INVENTION

Accordingly, the invention provides a distributed optical fibre sensorfor determining one or more parameters as functions of position along aplurality of sensing optical fibres from properties of probe lightbackscattered within the sensing optical fibres, for example to monitorthe environment along each sensing optical fibre, the sensor comprising:

a probe light source arranged to generate probe light pulses each at oneof a plurality of optical wavelengths; a coupler arranged to receive theprobe light pulses from the probe light source and to route pulses ofeach wavelength into a different corresponding one of said sensingoptical fibres, and to collect probe light backscattered within thesensing fibres; a detector arranged to receive the collectedbackscattered light and to separately detect light of each of saiddifferent wavelengths in said collected backscattered light; and ananalyser arranged to determine said one or more parameters as functionsof position along each of the sensing fibres from said detectedbackscattered light of each corresponding wavelength.

Such a sensor can make use of optical pathways and components common toall of the wavelengths to reduce the size, complexity and cost ofimplementing interrogation of multiple sensing fibres.

The invention can also be applied to a single loop of sensing fibre,with two interrogated pathways being opposite directions around the sameloop. Accordingly, the invention also provides a distributed opticalfibre sensor for determining one or more parameters as functions ofposition in both directions along a loop of sensing optical fibre fromproperties of probe light backscattered within the sensing opticalfibre, the sensor comprising: a probe light source arranged to generateprobe light pulses each at one of two optical wavelengths; a couplerarranged to receive the probe light pulses from the probe light sourceand to direct pulses of each wavelength into the sensing optical fibrein a different corresponding direction, and to collect probe lightbackscattered within the sensing optical fibre; a detector arranged toreceive the collected backscattered light and to separately detect lightof each of said different wavelengths in said collected backscatteredlight; and an analyser arranged to determine said one or more parametersas functions of position in each direction along the sensing opticalfibre from said detected backscattered light of each correspondingwavelength. When applied to a loop in this way, a single break in thesensing fibre can be accommodated without loss of function byinterrogating the fibre in each direction as far as the break.

The coupler may be a wavelength multiplexer/demultiplexer componentarranged to receive the probe light pulses of all of the plurality ofoptical wavelengths, and to demultiplex each wavelength onto a differentsensing fibre or different direction around the fibre loop. A singleconnector waveguide maybe used to deliver the pulses of all of thewavelengths to the coupler and to carry the collected backscatteredlight of all of the wavelengths from the coupler for delivery to thedetector.

The probe light pulses may be conditioned using one or more sourceoptical conditioning components through which the probe light pulses ofall of the plurality of wavelengths are passed before being launchedinto the sensing optical fibre or fibres. Similarly, the backscatteredlight may be conditioned using one or more detector optical conditioningcomponents through which all of the collected backscattered light ispassed before being detected. Such optical conditioning components mayinclude optical amplifiers, bandpass filters and similar.

The probe light pulses of the plurality of wavelengths may be generatedusing a switched wavelength laser, or using separate correspondinglytuned laser sources, or by applying wavelength shifting techniques tothe light from a single, master laser source. In particular, the probelight source may be arranged to generate the probe light pulses suchthat backscattered light of at least two of the plurality of wavelengthsis detected by the detector at the same time. Note that a single probelight pulse of a few nanoseconds duration launched into a sensingoptical fibre will typically give rise to backscattered light spreadover a few microseconds, depending on the length of the sensing opticalfibre. In order to increase the proportion of time within which eachsensing fibre is interrogated, the probe light source may therefore bearranged to generate the probe light pulses such that backscatteredlight of more than one, and optionally all of the plurality ofwavelengths at least partially overlaps at the detector.

The parameter may be a parameter of the environment around the sensingfibre. The sensor may be used to detect a variety of parameters, forexample vibration, temperature, pressure, and strain at the sensor fibreor fibres. The analyser may be arranged to determine the same parameterin respect of each of said sensing fibres or loop direction, or todetermine different parameters on some or all of the fibres or loopdirections. Such parameters may be detected using a variety of opticaltechniques which are known in the art. The invention may, for example beimplemented such that the detector detects coherent Rayleigh noise,Rayleigh backscatter, Raman scattering or Brillouin scattering at one ormore of said plurality of wavelengths, and the analyser determines oneor more of said parameters from properties of the coherent Rayleighnoise, Rayleigh backscatter, Raman scattering or Brillouin scattering.

The invention may be used to monitor a variety of environments andstructures, such as oil, gas and other geological wells, along aplurality of branches of such wells, pipelines, building structures, andalong security perimeters.

The invention also provides methods corresponding to the apparatusdiscussed above, for example a method of operating a distributed opticalfibre sensor to determine one or more parameters as functions ofposition along a plurality of sensing optical fibres from properties ofprobe light backscattered within the sensing optical fibres, to therebymonitor the environments of said sensing optical fibres, comprising:operating a probe light source to generate probe light pulses each atone of a plurality of optical wavelengths; coupling the probe lightpulses of each wavelength into a different corresponding one of saidsensing optical fibres; collecting probe light backscattered within thesensing optical fibres; separately and simultaneously detecting light ofeach of said different wavelengths in said collected backscatteredlight; and determining said one or more parameters as functions ofpositions along each of the sensing optical fibres from said detectedbackscattered light of each corresponding wavelength. The invention maysimilarly be applied as a method to a loop of sensing fibre withdifferent probe light pulse wavelengths being used in each directionaround the loop.

The probe light pulses may be delivered to the sensing optical fibre orfibres and the backscattered probe light may be collected and combinedfrom the sensing optical fibre or fibres by a wavelengthmultiplexer/demultiplexer component. The timing of the generation ofprobe light pulses by the probe light source may be controlled such thatat least some of the collected backscattered light contains light ofmore than one of said wavelengths.

The steps of operating the probe light source and detecting thebackscattered light may be implemented such that coherent Rayleigh noiseis detected in the backscattered light, and the step of determiningcomprises determination of said one or more parameters from propertiesof the coherent Rayleigh noise. The one or more determined parametersmay include a parameter representative of vibration in the one or morecorresponding sensing optical fibres.

In implementing any of the aspects mentioned above, the invention mayalso provide a method of determining parameters along a plurality ofextended paths, comprising disposing a sensing optical fibre along eachof said paths, coupling the sensing optical fibres to a singleinterrogator unit, using the interrogator unit to launch probe lightpulses of a different wavelength along each of said sensing opticalfibres, and using the interrogator unit to detect and analyse the probelight backscattered within the sensing optical fibres to determine saidparameters. This method may also be applied to interrogating in oppositedirections along a loop of sensing optical fibre.

BRIEF SUMMARY OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, of which:

FIG. 1 illustrates a distributed optical fibre sensor embodying theinvention;

FIGS. 2 a, 2 b and 2 c show some geological well applications for use ofthe invention;

FIG. 3 shows an embodiment of the invention using a different sensingoptical fibre disposed in each direction along an elongate structuresuch as a pipeline;

FIG. 4 shows the invention applied to a loop of sensing optical fibre;and

FIG. 5 shows how the sensor of FIGS. 1 to 4 can be implemented in moredetail.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to FIG. 1 there is illustrated a distributed optical fibresensor arranged to sense one or more physical parameters as a functionof position along part or all of each a plurality of sensing opticalfibres 10, 11, 12 using time domain reflectometry, or anotherreflectometry technique. An interrogator unit 5 of the sensor includes aprobe light source 14 for generating probe light pulses of suitabletimings, shapes and wavelengths, a detector 16 for detecting probe lightresulting from the probe light pulses being backscattered within thesensing fibres 10, 11, 12, and an analyser 18 for processing data, suchas properties of the backscattered and detected light, which has beenreceived from the detector.

The probe light source forms probe light pulses, each pulse having anoptical wavelength selected from a plurality of different wavelengths.Three such wavelengths are used in the figure, shown as λ₁, λ₂ and λ₃.The probe light pulses are forwarded to an optical circulator 20 andfrom there on to an optical coupler 22. The coupler provides awavelength multiplex/demultiplex function, delivering probe light pulsesof each of the wavelengths to a corresponding one of the sensing fibres10, 11, 12 as shown in the figure. In FIG. 1 the circulator 20 is shownas forming part of the interrogator unit 5, and the coupler as beingexternal to the interrogator unit. However, other arrangements may beused as appropriate to the implementation, for example by including thecoupler within the interrogator unit 5.

Probe light backscattered within the sensing fibres 10, 11, 12, iscombined together at the coupler 22, and delivered from there to thecirculator 20 which passes the collected light on to the detector 16.Conveniently, the probe light pulses and collected backscattered lightmay be coupled between the circulator 20 and coupler 22 using a singleconnector waveguide 24, typically a single optical fibre, and each ofthe probe light source and the detector may similarly be linked to thecirculator using single optical fibres.

The analyser 18 outputs analysis results such as a determination of theone or more physical parameters, and in FIG. 1 this output is passed toa computer display 26, although various other types of output mechanismmay be used. The analyser 18 may also use data derived from the detectedbackscatter to provide control signals 23 to the probe light source 14.A variety of control signals may be provided, including signalscontrolling the durations, timings and wavelengths of probe lightpulses, as required. In alternative embodiments, the control functionsmay be implemented separately to the analyser 18, for example in aseparate controller element (not shown in the figure).

The probe light source is arranged to generate the probe light pulses ofdifferent wavelengths according to particular timing schemes. Because adifferent wavelength is used to interrogate each sensing optical fibre,it is not necessary to wait until all the backscattered light of onepulse has arrived at the detector before launching another probe lightpulse. Pulses of different wavelengths may be generated at the same timeso as to overlap, or so that backscattered light from one or more pulsesof different wavelengths, or pulses of all of the wavelengths, overlapat the detector. In particular, the detector may be arranged toseparately detect backscattered light of each of the wavelengths, andmore particularly to separately and simultaneously detect backscatteredlight of more than one or of all of the wavelengths.

The probe light source contains one or more laser sources 30 to generatethe probe light pulses. For example, the pulses of all of the differentwavelengths may be generated using a single switched wavelength lasersource, suitably controlled, or pulses of the different wavelengths maybe generated using discrete separate laser sources, or using a singlelaser with a wavelength shifting method. The probe light pulses areconditioned in the probe light source by one or more source opticalconditioning components 32. If multiple laser sources are used then someof these components may be specific to one or more of the lasers, buttypically one or more of the source optical conditioning components willbe used to condition pulses of all of the wavelengths.

The detector 16 preferably uses a different detector element 36 todetect backscattered light of each of the different probe lightwavelengths. The detector elements may be, for example, suitablephotodiodes, with wavelength selective components used to direct thecollected light to the appropriate detector. The backscattered light isconditioned in the detector using one or more detector opticalconditioning components, and typically at least one of the detectoroptical conditioning components is used to condition backscattered lightof all of the generated wavelengths. Use of a common optical pathway andcomponents common to all of the wavelengths simplifies the interrogatorand reduces costs.

The optical conditioning components in the interrogator, includingindividual components which are used to condition light of all of thewavelengths, may include optical amplifiers, band pass filters, andother components.

The sensing fibres may all be of the same type, or may be of differenttypes including, without limitation, single mode fibre, multimode fibre,fibre with high birefringence, and fibre especially adapted or encasedso as to respond or enhance the response to changes in one or more ofpressure, temperature, and other parameters which are to be measured.

A variety of interrogation techniques may be used, for example dependingon the parameters which are to be measured, and the probe light source14, detector 16, sensing fibres and other components of the sensor maybe adapted accordingly. For example, the sensing fibres may beinterrogated using techniques based on Rayleigh backscatter, coherentRayleigh noise, Raman scattering, and Brillouin scattering. In someembodiments all of the sensing fibres are interrogated using the sametechnique, or the same combination of techniques, and in otherembodiments different techniques or combinations of techniques are usedon some or all of the sensing fibres. The sensor may use selected onesof these techniques as appropriate to measure parameters such asvibration, static or transient strain, temperature, and pressure.

The arrangement of FIG. 1 can be used in a variety of situations. FIG. 2a illustrates the sensor being used to monitor multiple branches of asingle geological borehole 40, such as an oil or gas well. Each branch42 is monitored using a different one of the sensing fibres. In FIG. 2 bmultiple sensing fibres are used to monitor the same borehole 40, forexample to provide redundancy, or to use different sensing fibres tosense different parameters. In FIG. 2 c multiple sensing fibres are usedto monitor separate boreholes 40.

In other applications, the multiple sensing fibres are used to monitoralong different pathways across or through a building structure orinfrastructure component, such as a bridge, a pipe line or a roadway.FIG. 3 a illustrates use of the invention to increase the length of apipeline 42 which can be monitored using a single interrogator unit 5,by directing sensing optical fibres in opposite directions along thepipeline from the interrogator unit. A similar arrangement can be usedfor other extended structures, or for security/intrusion monitoringacross long stretches of land or fences and other types of borders.

In FIG. 4, an open loop 46 of sensing optical fibre is disposed throughan environment to be sensed, for example along a borehole, fence, alongthe ground etc. Probe light pulses of one optical wavelength arelaunched into the loop 46 in one direction, and pulses of a secondwavelength are launched into the loop 48 in the opposite direction. Fromthe point of view of the coupler 22 and interrogator 5 the loop appearsto be two separate fibres extending individually through theenvironment. However, the loop topology allows interrogation along thefull length of the sensing fibre to continue even when the loop isbroken in one place.

FIG. 5 shows details of optics and electronics which may be used forputting into effect the distributed optical fibre sensor describedabove.

Two separate optical sources 50 a and 50 b are shown within the probelight source 14, each optical source emitting a narrow band ofwavelengths centred at λ_(a) and λ_(b) respectively. It wouldalternatively be possible to use a single, wavelength switched source,or a combination of sources with switchable and fixed wavelengths. Sincetwo separate optical sources are shown in this embodiment, a wavelengthcombiner component 114 is required to route the two signal wavelengthsonto a common optical path. If a wavelength switched source were to beused alone, then this component would not be required. For convenienceof implementation, the wavelengths λ_(a) and λ_(b) used in the systemcould lie within the operating band of typical erbium-doped fibreamplifiers, between 1528 nm to 1562 nm, and the optical sources could bedistributed-feedback laser diodes.

Once combined, the two signal wavelengths are then fed through anoptical conditioning chain 116 whose function is to amplify the light tosuitable power levels and to provide optical filtering to avoid thedeleterious effects of amplified spontaneous emission (ASE) from theamplifier elements. This optical conditioning chain comprises opticalcomponents 32 through which probe light pulses of both pulse wavelengthspass. Typically, peak powers of the order of 1 W might be delivered tothe sensing fibres 10, 11 and the ASE suppression bandwidth might be˜0.2 nm. Light emerging from the optical conditioning chain 116 isdirected to the optical circulator 20 that serves to route probe lightfrom the probe light source 14 to the coupler 22 which is provided by awavelength multiplexer/demultiplexer component. The coupler 22 couplesthe probe light pulses of each wavelength into a different one of thesensing fibres 10, 11 and light from the sensing fibres back to thecirculator 20 which couples the light from the sensing fibres to thedetector 16.

Backscattered light returning from the sensing fibres through thecoupler and circulator is directed into the detector 16, and inparticular into an optical signal conditioning chain 120. This chaincontains further amplification and filtering components 38 required toincrease the received signal powers to levels suitable for low-noisedetection. Following passage through the signal conditioning chain 120,the two signal wavelengths λ_(a) and λ_(b) are separated by thewavelength de-multiplexing component 122. After separation, the twosignal wavelengths λ_(a) and λ_(b) are each further filtered to a narrowband using components 124 a, 124 b and 126 a, 126 b respectively. Inthis embodiment, the narrow band filters are fibre Bragg gratings withapproximately 80 pm reflection bandwidth. Finally, each wavelength isreceived by its own photodetector, 130 a and 130 b respectively.Conveniently, PIN photodiodes may be used for this purpose.

The signals from each photodetector 130 a, 130 b are digitized by thedata acquisition unit 134 and fed to the analyser 18, which controls theoptical sources 110 a and 110 b via driver circuits 150 a and 150 b.Apart from providing accurately timed electrical pulses to the opticalsources to control probe light pulse timing and length, these drivercircuits may also serve to fine-tune the wavelength of the opticalsources. This can be achieved, for example, by control of lasertemperature. In possible alternative embodiments, fine-tuning of thecentre wavelength of different probe light pulses might be accomplishedby controlled filtering either before the unconditioned probe lightenters the wavelength combiner 114 or after leaving the wavelengthdemultiplexer 122. In the latter case, the centre wavelength of one orboth of the fibre Bragg gratings could be thermally tuned. In anotherpossible embodiment, fine control of wavelength might alternatively beachieved by phase or frequency modulation of light using a radiofrequency optical modulator together with appropriate filtering.

Based on analysis of the backscattered probe light, the analyser 18provides control signals to the driver circuits 150 a, 150 b, forexample to control probe light pulse length and probe pulse wavelengthas required.

Although various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example only, and not limitation. For example, the skilledperson will appreciate that the optical, electronic and data processingfunctionality of the distributed optical fibre sensor can be implementedand distributed across different functional or physical units in variousways according to convenience and implementation objectives.

It will be understood by those skilled in the relevant art(s) thatvarious changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

The invention claimed is:
 1. A distributed optical fibre sensor fordetermining one or more parameters as functions of position along aplurality of sensing optical fibres from properties of probe lightbackscattered within the sensing optical fibres, the sensor comprising:a probe light source arranged to generate probe light pulses each at oneof a plurality of optical wavelengths; a wavelengthmultiplexer/demultiplexer component arranged to receive the probe lightpulses from the probe light source and to direct pulses of eachwavelength into a different corresponding one of said sensing opticalfibres, and to collect probe light backscattered within the sensingoptical fibres; a detector arranged to receive the collectedbackscattered light and to separately detect light of each of saiddifferent wavelengths in said collected backscattered light; and ananalyser arranged to determine said one or more parameters as functionsof position along each of the sensing optical fibres from said detectedbackscattered light of each corresponding wavelength.
 2. The sensor ofclaim 1 wherein the wavelength multiplexer/demultiplexer component isarranged to receive the probe light pulses of all of the plurality ofoptical wavelengths along a connector waveguide and to output all of theplurality of optical wavelengths of the collected backscattered lightalong the connector waveguide.
 3. The sensor of claim 1 wherein theprobe light source is arranged to generate said probe light pulses suchthat backscattered light of at least two of the plurality of wavelengthsis detected by the detector at the same time.
 4. The sensor of claim 1wherein the probe light source is arranged to generate said probe lightpulses such that backscattered light of all of the plurality ofwavelengths is detected by the detector at the same time.
 5. The sensorof claim 1 arranged such that the detector detects at least one ofRayleigh, Raman and Brillouin backscatter at each of said plurality ofwavelengths, and the analyser determines said one or more parametersfrom said at least one of Rayleigh, Raman and Brillouin backscatter. 6.The sensor of claim 1 arranged such that the detector detects coherentRayleigh noise at one or more of said plurality of wavelengths, and theanalyser determines one or more of said parameters from properties ofthe coherent Rayleigh noise.
 7. The sensor of claim 1 wherein each ofthe one or more parameters represents one of: vibration; temperature;pressure; and strain.
 8. The sensor of claim 1 wherein the analyser isarranged to determine the same parameter in respect of each of saidsensing fibres.
 9. The sensor of claim 1 wherein the probe light pulsesare conditioned using one or more source optical conditioning componentsthrough which the probe light pulses of all of the plurality ofwavelengths are passed before being launched into the sensing opticalfibre or fibres.
 10. The sensor of claim 9 wherein one or more of theoptical conditioning components include at least one of: an opticalamplifier; and a bandpass filter.
 11. The sensor of claim 1 wherein thebackscattered light is conditioned using one or more detector opticalconditioning components through which all of the collected backscatteredlight is passed before being detected.
 12. The sensor of claim 1 furthercomprising said plurality of sensing fibres, the sensing fibre(s) beingdisposed in or along at least one of: a well; a plurality of branches ofa well; a pipeline; a building structure; a security perimeter; and inopposite directions along an elongate pathway.
 13. The sensor of claim 1wherein probe light pulses of the plurality of wavelengths are generatedusing a switched wavelength laser.
 14. The sensor of claim 1 wherein theprobe light pulses of each of the plurality of wavelengths are generatedusing a separate correspondingly tuned laser.
 15. A method of operatinga distributed optical fibre sensor to determine one or more parametersas functions of position along a plurality of sensing optical fibresfrom properties of probe light backscattered within the sensing opticalfibres, comprising: operating a probe light source to generate probelight pulses each at one of a plurality of optical wavelengths; couplingthe probe light pulses of each wavelength into a different correspondingone of said sensing optical fibres; collecting probe light backscatteredwithin the sensing optical fibres; separately detecting light of each ofsaid different wavelengths in said collected backscattered light; anddetermining said one or more parameters as functions of position alongeach of the sensing optical fibres from said detected backscatteredlight of each corresponding wavelength, wherein the probe light pulsesare delivered to the sensing optical fibres and the backscattered probelight is collected and combined from the sensing optical fibres by awavelength multiplexer/demultiplexer component.
 16. The method of claim15 comprising controlling the timing of the generation of probe lightpulses by the probe light source such that at least some of thecollected backscattered light contains light of more than one of saidwavelengths.
 17. The method of claim 15 comprising separately andsimultaneously detecting backscattered light of all of said differentwavelengths.
 18. The method of claim 15 comprising determining said oneor more parameters from properties of at least one of Rayleigh,Brillouin and Raman backscatter of said probe light pulses.
 19. Themethod of claim 15 wherein the steps of operating the probe light sourceand detecting the backscattered light are arranged such that coherentRayleigh noise is detected in the backscattered light, and the step ofdetermining comprises determining said one or more parameters fromproperties of the coherent Rayleigh noise.
 20. The method of claim 15wherein the one or more parameters include a parameter representative ofvibration in the one or more corresponding sensing optical fibres.